Designers and inventors have sought to develop a light modulator which can operate alone or together with other modulators. Such modulators should provide high operating speeds (KHz frame rates), a high contrast ratio or modulation depth, have optical flatness, be compatible with VLSI processing techniques, be easy to handle and be relatively low in cost. Two such related systems are found in U.S. Pat. Nos. 5,311,360 and 5,841,579 which are hereby incorporated by reference.
According to the teachings of the '360 and '579 patents, a diffractive light modulator is formed of a multiple mirrored-ribbon structure. An example of such a diffractive light modulator 10 is shown in FIG. 1. The diffractive light modulator 10 comprises elongated elements 12 suspended by first and second posts, 14 and 16, above a substrate 20. The substrate 20 comprises a conductor 18. In operation, the diffractive light modulator 10 operates to produce modulated light selected from a reflection mode and a diffraction mode.
FIGS. 2 and 3 illustrate a cross-section of the diffractive light modulator 10 in a reflection mode and a diffraction mode, respectively. The elongated elements 12 comprise a conducting and reflecting surface 22 and a resilient material 24. The substrate 20 comprises the conductor 18.
FIG. 2 depicts the diffractive light modulator 10 in the reflection mode. In the reflection mode, the conducting and reflecting surfaces 22 of the elongated elements 12 form a plane so that incident light I reflects from the elongated elements 12 to produce reflected light R.
FIG. 3 depicts the diffractive light modulator 10 in the diffraction mode. In the diffraction mode, an electrical bias causes alternate ones of the elongated elements 12 to move toward the substrate 20. The electrical bias is applied between the reflecting and conducting surfaces 22 of the alternate ones of the elongated elements 12 and the conductor 18. The electrical bias results in a height difference between the alternate ones of the elongated elements 12 and non-biased ones of the elongated elements 12. A height difference of a quarter wavelength λ/4 of the incident light I produces maximum diffracted light including plus one and minus one diffraction orders, D+1 and D−1. In the diffraction mode, the diffractive modulator forms an optical structure similar to a square well grating.
FIGS. 2 and 3 depict the diffractive light modulator 10 in the reflection and diffraction modes, respectively. For a deflection of the alternate ones of the elongated elements 12 of less than a quarter wavelength λ/4, the incident light I both reflects and diffracts producing the reflected light R and the diffracted light including the plus one and minus one diffraction orders, D+1 and D−1. In other words, by deflecting the alternate ones of the elongated elements 12 less the quarter wavelength λ/4, the diffractive light modulator 10 produces a variable reflectivity.
Unfortunately, when arbitrarily polarized light impinges on the diffractive light modulator depicted in FIGS. 2 and 3, different polarization states interact with the diffractive light modulator differently. As depicted in FIG. 4, at a given instant in time, any arbitrarily polarized light incident upon the diffractive modulator, or grating, can be decomposed into two components; one where the electric field of the light is parallel to the ribbons, or grating grooves, henceforth referred to as P, and another where the electric field of the light is perpendicular to the ribbons, or grating grooves, henceforth referred to as S. Two polarization states are deemed to be different if the ratio of the P and S components for the two states is different. The electric field of the P component initiates an oscillation in the electrons or dipole of a reflector along the ribbon length, and the S component initiates an oscillation in the electrons or dipoles of the reflector in a direction perpendicular to the ribbons. When the oscillating electrons or dipole returns to its original state, it emits, or scatters, light back. The extent of light scattered back depends on the extent of the induced oscillation, which in turn depends on the wavelength of light, the proximity of the oscillating electrons or dipole to the physical boundary of the material, and the fact that the electric field has to be continuous at the material boundaries. For the predominant effect, the oscillations induced by the electric field of the P component interact differently with the ribbon edge than the oscillations induced by the electric field of the S component. This results in a different extent of back scattering for P and S components. Therefore, different polarization states with different ratios of P and S components exhibit different amounts of loss for the light reflected or diffracted back. This leads to Polarization Dependent Losses (PDL) in which one polarization state is attenuated more than the other.
For telecommunications applications where the polarization state at the input of a device is not guaranteed and changes with time, PDL causes the intensity of light at the output of the device to vary with time. This results in the degradation of the quality of the transmission and therefore PDL in the device must be minimized. What is needed is a diffractive light modulator with an output response that is as independent of the polarization state as possible.